Flame sensitive device



Dec. 24, 1946-. S, G, E KlN 2,412,977

FLAME SENSITIVE DEVICE Filed Nov. 19, 1945 Chromz'zed y INVENTOR Samuel 6.51%?!) B Y I W 7 %M4/M 15 ATTORNEY Patented Dec. 24, 1946 FLAME SENSITIVE DEVICE Samuel G. Eskin, Pittsburgh, Pa., assignor to Robertshaw Thermostat Company, Youngwood, Pa., a corporation of Pennsylvania Application November 19, 1943, Serial No. 510,851

2 Claims.

This invention relates to electric circuit elements having the characteristics of largeresistance variation under temperature changes and, more particularly, to metallic flame sensitive resistors.

. Several metals and metal alloys have large positive temperature coefiicients of electrical resistivity rendering them valuable in various-circuit applications. These circuit elements may i be coils of wire, generally termed ballast resistors, and in order to utilize them to the fullest advantage it is sometimes desirable to expose them directly to the heat of a flame. In one application the coil is placed in the flame of a gaseous fuel burner and by its consequent change in resistance serves as a safety device to shut off the flow of fuel should the flame become accidentally extinguished.

Of the several alloys which disclose suitable electrical properties for the purpose described some will resist oxidation when exposed to the flame of burning natural gases. The usefulness of such materials is limited, however, by the corrosion to which they are subject when heated by manufactured gases containing sulphur. Various metallic and non-metallic coatings for metals are known which offer more or less protection against corrosion from sulphurous atmospheres but, in the particular services to which this application relates, it is of the utmost importance that the coated material retain the electrical re-. sistance properties which the uncoated material possessed.

It is an object of this invention to protect a metallic flame sensitive resistor from corrosion without adversely affecting its electrical resistance properties.

Another object of the invention is to permit the coated material to be bent or formed into a coil or other desired shape and to be repeatedly heated and cooled without cracking or peeling of the coating from the base material.

Another object of the invention is to permit formed coils of wire to be coated uniformly.

Other objects and advantages will become apparent from the following description taken in connection with the accompanying drawing, wherein:

Fig. 1 is aschematic view of a safety system embodying the flame sensitive device of this invention; and

Fig. 2 is an enlarged sectional view of the device.

Referring more particularly to the drawing, the flame sensitive device It! is shown as exposed to the flame of a gaseous fuel burner 12 connected with a main fuel pipe I! through which the flow of gaseous fuel is controlled by an electromagnetically operated valve l6. As is customary in such devices, the electromagnetic valve l6 may be opened by energization of its coil 23 and closed by gravity upon deenergization thereof.

Electric current for operating the electromagnetic valve l6 may be supplied from line wires connected to the primary of a transformer IS. The valve circuit may be traced from the secondary of the transformer l8 by wire 20, wire 22, coil 23, wire 24, and wire 26 back to the secondary of the transformer I8. A control switch (not shown) may be incorporated in the described circuit to control the energization of the coil 23, as desired. In order to cause movement of the electromagnetic valve IE to closed position when the burner flame is accidentally extinguished, the flame sensitive device In is connected in shunt with the coil 23 by wires 28 and, connected with wires 24 and 22, respectively.

The flame sensitive device l may consist of a coil of wire from material having normally a relatively low resistance and a high positive temperature coeflicient of resistance. In this embodiment the device I0 is formed from nickelsteel wire containing about 70% nickel-30% I iron. The electrical resistance characteristics of the flame sensitive device in so formed are such that when heated by the flame of the burner l2 sufficient current will flow in the coil 23 to maintain the valve IS in open position, but when unheated by the burner flame, insufficient current passes through the coil 23 to maintain the valve IS in open position.

In the burner control apparatus described, the flame sensitive device It is used to determine whether the electromagnetic valve I6 is maintained in open position after running conditions have been established. In order to establish such conditions it is necessary that the valve l6 be initially moved to open position in order that fuel may flow to the burner l2 and the flame thereof heat the device ill to bring the'safety features into operation. Such movement of the valve [5 may .be accomplished by a manually operated lifting device 32 attached to the solenoid (not shown) of the electromagnetic valve l6 so that in the starting condition (while the flame sensitive device 10 is unheated by the burner flame) the valve l6 may be manually opened and maintained in such position until the flame sensitive device 10 becomes sufficiently heated to render such manual actuation unnecessary. It will be 2&19377 3 F apparent, however, that a constant burning pilot to which the device 90 would be exposed, or other arrangements known to the art, could be employed without affecting the scope of this invention.

As previously indicated, this invention is more.

particularly directed to protecting the flame sensitive element It from corrosion in the sulphur-.

ous atmosphere of the burner it without sacrifice. of the desired electrical resistance properties thereof. Accordingly, the nickel steel wire of which the device it is formed is provided with a protective coating which has been found to be highly resistant to corrosion in suiphurous atmospheres as may exist when manufactured gases are used as the fuel.

The protective coating for the device it) of this invention is applied by a process known as chromizing, whereby the nickel-steel underbody is surface-alloyed with chromium by a preferred method now to be described. The coil of nickelsteel wire forming the underbody of the flame sensitive device 50 is packed in a. mixture containing equal parts of ferrochrome and 60-mesh alumina and placed in a suitable furnace. No special surface treatment need be given the coil before the chromizing is started. -The atmosphere of the furnace is rendered non-oxidizing by the use of a gas such as hydrogen and hydrogen chloride. Heating of the coil is carried on for eight hours at 1000 C. which has been found sufficient to form a layer of adequate thickness. The duration of the treatment may, however, be varied in accordance with the thickness of surface layer desired.

A flame sensitive device it so treated has a surface coating in the form of a difiused layer integral with the nickel-steel underbody and the chromium content of the layer appears to decrease from the surface inwardly. The layer generally has a wall thickness from 0.0001" to 0.0002" and the device so formed shows excellent heat and corrosion resistance under severe conditions. The device also has the advantage of capability of being bent, wound or otherwise formed without cracking or peel= ing of the coating from the underbody and will undergo expansion and contraction with similar results. It appears, therefore, that while the exterior surface exposed to the flame of the burner i2 may consist of pure chromium, the general character of the coating consists of a chromium-nickel-iron alloy intimately bonded with the nickel-steel wire forming the underbody and.

having the characteristics of all metallic coatings formed by difiusion of metals.

The flame sensitive resistor thus formed is peculiarly adapted for the purpose described by reason of the retention of its electrical resistance properties substantially unchanged, although the uncoated nickel-steel wire which possessed such properties is new highly resistant to corrosion and heat under the severe conditions to which it is exposed. The invention thus serves the purpose of a fiamesensitive device for use in the burner control apparatus described in connection with Fig. 1 and will not only remain effective for a much greater length of time than an uncoated coil but will have none of the desired properties adversely affected by reason of the protective coating.

While the chromizing process has been outlined herein and satisfactory cells may be formed by use of such process, it will be apparent that the principles of this invention are not limited to flame sensitive devices produced by following the exact steps set forth. Other methods including,

variations of the described process may be em.- ployed to produce the desired difiused layer on a flame sensitive electrical resistor without depar-i ture from the spirit of the invention as set forth in the appended claims.

Iclaim:

1. A flame sensitive electrical resistor comprising a wire coil consisting of a chromium-free nickel-iron alloy of normally a relatively low electrical resistance value and a high positive temperature coefficient of resistance value having a chromium-containing layer integrally diffused with the surface thereof which is exposed to the flame, said layer being adapted to protect said wire from corrosion in sulphurous atmospheres resulting from the presence of the flame and without substantial alteration in the said electrical resistance value.

2. A flame sensitive electrical resistor comprising a wire coil consisting of about nickel-.- 30% iron alloy which is chromium-free and has normally a relatively low electrical resistance value and a high positive temperature coemcient of resistance value, said wire having a chromium containing layer integrally difiused with the surface thereof which is exposed to the flame, said layer being adapted to protect said wire from corrosion in sulphurous atmospheres resulting from the presence of the flame and without substantial alteration in the said electrical resistance value.

SAMUEL G. ESKIN. 

